In the above-identified copending applications, the latent electrostatic image, formed in a manner known to the art, is developed by a liquid developing composition. The liquid developer usually comprises a dielectric liquid, such as a low-boiling aliphatic hydrocarbon, in which are dispersed pigmented particles. These particles are conventionally formed of carbon black associated with a polymer. These pigmented particles are referred to as "toner particles". They are normally charged to a polarity opposite to the charge of the latent image, so that they will move to the image by electrophoresis to develop the same. In the copending applications, above identified, there have been disclosed various methods and forms of apparatus for forming a gap between the carrier sheet and the developed electrophotographic image, across which gap the image is to be transferred. There has been pointed out that, in the prior art, part of the carrier liquid in the non-image areas will be absorbed by the carrier sheet and must be dried, usually by heat. This evaporates hydrocarbons into the circumambient atmosphere. The amount of evaporation permitted is strictly controlled by law. This limits the speed at which the electrophotographic copying machine can be operated. A non-toxic, light, paraffinic hydrocarbon carrier liquid, such as ISOPAR-G (trademark of Exxon Corporation), is one of the aliphatic hydrocarbon liquids which is used in the developing composition. The contacting of a carrier sheet with the freshly developed image may induce smudging, smearing, or squashing of the developed image. This reduces the resolution. Then too, the charge of the toner particles is opposite to the charge of the latent electrostatic image. This arrangement is such, in the prior art, that the paper tends to stick to the photoconductive, or insulating, surface on which the image is developed. This produces difficulty in removing the carrier sheet bearing the developed image from the photoconductive surface. The usual carrier sheet is paper, and repetitive contact of paper with a moist developed image leaves paper fibers on the photoconductive surface. Since all of the developed image is rarely transferred to the carrier sheet, the paper fibers left behind contaminate the developing liquid.
We have found, as pointed out in the copending applications, above identified, that these disadvantages can be avoided by spacing the carrier sheet from the photoconductor to form a gap and causing the freshly developed image to negotiate the gap between the photoconductor and the carrier sheet by placing a charge on the back of the carrier sheet by means of a corona or the like.
In copending application Ser. No. 149,539, now U.S. Pat. No. 4,364,661, there is described the method of transferring freshly liquid-developed images across a gap. Methods are disclosed of forming a gap by providing the carrier sheet with protuberances formed on the carrier sheet which prevent the contact of the major area of the carrier sheet with the freshly developed image by deforming the sheet or otherwise forming protuberances thereon. In our copending application Ser. No. 249,336, now U.S. Pat. No. 4,378,422, there is disclosed another means of carrying out a gap transfer method. We there provide spacing particles to form the desired gap between the substrate bearing the freshly developed electrostatic image by positioning them on the developed image or by forming spacing protuberances on the photoconductive, or insulating, surface on which the latent electrostatic image is formed.
The developed image, in its transfer across the gap, passes in small geyser-like columns. This is occasioned by the imposition of a field behind the carrier sheet of a polarity opposite to the polarity of the toner particles and of a higher potential than the charge of the latent image. The columns of the developed image, in striking the carrier sheet, form dots. The spacing of these dots varies with the density of the image. This accounts for the gray scale which is achieved by our gap-transfer process. Unfortunately, when copying on rough paper, the dots, while sufficiently dense--that is, thick--will not fill the valleys between the peaks present in a rough-surfaced paper. The result is that a comparatively poor copy appears on a carrier sheet having a rough surface.